1. Field of the Invention
This invention relates to a semiconductor light-emitting device and, in particular, to a high-brightness semiconductor light-emitting device with a transparent conductive film of metal oxide to serve as a current spreading layer.
2. Description of the Related Art
In recent years, the crystalline quality of GaN-based or AlGaInP-based semiconductors is enhanced since they can be grown by a MOVPE (metalorganic vapor phase epitaxy) method. Thus, a high-brightness blue, green, orange, yellow, and red light-emitting diode (herein referred to as LED) as a semiconductor light-emitting device can be fabricated.
However, in order to offer the high brightness, the current spreading property needs to be improved such that a current is uniformly supplied into a chip plane of an LED. For example, an AlGaInP-based LED device is fabricated such that the current spreading layer has a thickness increased to about 5000 nm to 10000 nm. When using such a thick current spreading layer, diffusion of the dopant will occur since the heat history during the crystal growth becomes long, resulting in deterioration of the active layer. In addition, the current spreading layer needs to have a low resistivity to spread current. Thus, it needs to be doped at a high concentration. However, the high-concentration doping leads to lowering of the transparency of the current spreading layer. Further, the growth of the thick current spreading layer causes an increase in the manufacturing cost of the LED device. Thus, the LED device is difficult to fabricate at low cost.
In consideration of this, a method is proposed that an ITO (indium tin oxide) or ZnO (zinc oxide) film is used as the current spreading layer to offer a sufficient translucency and good current spreading characteristics (JP-A-8-83927). Further, a method is proposed that an ITO film is directly formed on a p-type clad layer (U.S. Reissued Patent No. 35665).
Thus, when the ITO film is used as the current spreading layer, the conventional 5000 nm to 10000 nm thick current spreading layer, i.e., the corresponding epitaxial layer is no longer needed to be grown, and as a result, the above-mentioned problems can be solved.
However, when using the ITO film as the current spreading layer, there is a typical problem that a contact resistance occurs between the semiconductor layer and the metal-oxide ITO film, thus causing an increase in forward operating voltage. As a solution to this problem, a method is proposed that LED is driven at a low voltage by providing a contact layer with a high carrier concentration of 1.0×1019/cm3 or more between the semiconductor layer and the ITO film so as to form a tunnel junction between the contact layer and the ITO film (for example, U.S. Reissued Patent No. 35665). However, semiconductor materials that can stably realize the contact layer with a high carrier concentration of 1.0×1019/cm3 or more are limited. According to the research results of the inventors regarding the contact layer, Zn-doped AlxGa1-xAS (0≦X≦0.4) is an optimal semiconductor that can stably provide the contact layer with a high carrier concentration of 1.0×1019/cm3 or more. However, since AlGaAs is not transparent to the emission wavelength, it needs to be formed into a thin film with a thickness of about 30 nm or less.
p-type dopants for AlGaInP-based compound semiconductors can be beryllium (Be), magnesium (Mg), zinc (Zn) and the like. Among these, Be materials used in molecular beam epitaxy (MBE) can be doped at a high concentration in a reduced diffusion but suffer from the drawback of being highly toxic. Zn is widely used as a p-type dopant for AlGaInP-based or AlGaAs-based compound semiconductors, but its diffusion constant is relatively large. Therefore, it is known that when doping Zn into the p-type cladding layer and the p-type buffer layer, the Zn is diffused into the active layer due to the heat history so that the brightness and reliability of the LED is lowered. Consequently, it is preferable to use, as the p-type dopant, Mg with a relatively smaller diffusion constant than Zn.
However, as previously mentioned, since the semiconductor to stably realize the contact layer with a high carrier concentration of 1.0×1019/cm3 or more is only Zn-doped AlxGa1-xAS (0≦X≦0.4), Zn needs to be used as the p-type dopant for the contact layer. However, even in the contact layer located at the uppermost layer with the least heat history during the growth, Zn in the contact layer is diffused easily by the heat history during the growth and cooling. The diffusion of Zn, the p-type dopant for the contact layer, causes the following two negative effects.
The first negative effect is to cause the lowering of output of the LED element. The diffused dopant creates concentration diffusion in the depth direction of the LED element and, after being diffused into the active layer of the the center of non-radiative recombination, thereby lowering the LED element, forms an impurity level in the active layer to be emission output.
The second negative effect is to increase the operating voltage of the LED element. The carrier concentration of the contact layer is decreased by the diffusion of Zn dopant from the contact layer, so that the tunnel junction between the contact layer and the ITO film becomes difficult to form, causing an increase in the tunnel voltage. Consequently the operating voltage of the LED element must be increased.